The Secret History of the Mongols is a good primary source to start off with. There are plenty of translations. The best one for scholarly work is:

The Secret History of the Mongols: A Mongolian Epic of the Thirteenth Century, Igor De Rachewiltz, trans., ed., (Boston: Brill, 2004).

The best one for fun is:

The Secret History of the Mongols, Francis Woodman Cleaves, trans., ed., (Cambridge, MA: Harvard University Press, 1982)